Fungus gnats are a common pest that many experienced plant owners have encountered before. They are often seen flying around your plant's soil or lurking around standing water. Fungus gnats are extremely stubborn, so understanding their habits and cycles is your best weapon against them. 

Description 

They belong to the insect order Diptera. Often mistaken for mosquitoes or fruit flies, fungus gnats can be distinguished by their smaller size (usually around ⅛”). They are usually black or grey in colour with a small Y shaped vein on their wings. They have an erratic and slow flying path that is often near the soil of a houseplant. 

Signs and symptoms

Signs and symptoms besides the appearance of these bugs can be exhibited in the growth habits of your plant. Look out for new growth coming in damaged, shriveled, or stunted, and older growth yellowing or wilting. Keep in mind that fungus gnats are attracted to organic matter, so they are more likely to be drawn to your plant if it is already experiencing symptoms of root rot. 

Causes and lifecycle

Fungus gnats can come into your home through windows or by hitching a ride on a new houseplant. They have a short life cycle, which takes around 3-4 weeks to complete. Their adult life only lasts around 10 days. They lay their eggs in the soil and live egg-larvae- pupae all in the soil. When they emerge as the recognizable adult flies, they mate, lay eggs back in the soil, and restart this cycle over and over. An adult fungus gnat can lay 100-300 eggs in its very short lifespan. which is why these pests are a challenge to eradicate. It is in the larval stage that the most damage to your plant occurs. The growing lavae begin to feed on organic matter in the soil, and often end up eating the roots of your plant. This is where the damage to your plant's health begins to be noticeable.  Once in their adult stage, fungus gnats are not particularly harmful to the plant, except in perpetuating the lifecycle.

How to stop fungus gnats

It's best to target fungus gnats at all stages. There are many different methods, but these are the best-proven methods. 

BTI

BTI, which stands for Bacillus thuringiensis subspecies israelensis, also known as mosquito bit, is a highly effective method for killing fugnus gnats in their larval stage. Mosquito bit comes in different forms, so it's best to read the instructions of the product that you choose to purchase to ensure the directions are followed correctly. Typically, BTI will need to be soaked in water for around 30 minutes. If you are using the product loose, you can put it in a tea strainer for maximum efficiency. Pour the water in which the BTI has been steeped over your soil during scheduled waterings. BTI produces a crystal protein that, when consumed by the larvae, binds to their gut wall and prevents them from eating, inevitably killing them. If you are using BTI on an infected plant, be sure to repeat treatment for at least 3 weeks during your regularly scheduled watering cycle. At Scrim's, we recommend using BTI with waterings even when you don't have fungus gnats, as a preventative measure. 

Diatomaceous earth

Another effective treatment for fungus gnats, in both their larval and adult stage, is diatomaceous earth. This is a powdery substance made from fossilized aquatic organisms. Sprinkle a thin layer onto your infected plant's soil when the top of the soil is completely dry. As the fungus gnats emerge from the soil, they are exposed to this substance which scratches their exoskeletons and absorbs the waxy protective layer of their skin, causing them to dehydrate and die. 

Yellow sticky tape

Finally, to target the flying adult bugs, the easiest and most effective method is yellow sticky tape. Fungus gnats are attracted to the bright yellow because it mimics the wavelength of light associated with flowers and pollen. Leave a few sticky traps near the soil of your plant so that when they are flying around, the gnats will land and become trapped on the sticky surface. If you notice the sticky tape is collecting a large amount of adults, it's a sign that there is more going on with the plant than meets the eye. Consider changing the soil and taking a look at the roots. Remove the plant from its grow pot, remove soil, and cut away any damaged roots (look for soggy roots). Trim all mushy roots slightly above the point of mush (making sure to sanitize your tool between cuts). Place your plant back in its cleaned and sterilized grow pot with fresh soil. Think about the type of soil you are using and consider adding some orchid bark or cactus soil to ensure more aeration and better drainage in the soil. 

Plants at risk 

Note that some plants are more susceptible to fungus gnats, in particular plants that prefer moist conditions or lots of humidity, such as African violets, begonias, poinsettias, and ferns. Make sure that you are not letting plants sit in excess water and remember that sometimes the best treatment is prevention

In conclusion 

Understanding the life cycle of a fungus gnat, how they arrive, and how to treat them will help you to prevent and remove these pests from your AND your plants lives.  BTI, diatomaceous earth and sticky tape are an effective trifecta because they target the fungus gnats at various stages of their development, leaving little space for them to continue their lifecycle.